The invention falls within the field of radio systems.
More precisely, the invention relates to a registration method used for registering at least one dual mode mobile station (i.e. having two operating modes: xe2x80x9ccordlessxe2x80x9d and xe2x80x9ccellularxe2x80x9d) with an associated private base station, the base station together with the dual mode mobile station making up a cordless radio system.
First of all, it should be pointed out that there are two main families of radio system: cordless systems and cellular systems.
Conventionally, a cordless system (of the CT2 or DECT type for instance) comprises a private base station with which there are associated one or more mobile stations. There are many known applications of such a cordless system.
Thus, in a first application, the private base station is located in a dwelling, and the user and family have one or more mobile stations operating with the private base station. In this case, the private base station is also called a home or personal base station, and the mobile stations are also called portable handsets or terminals.
In a second application, the private base station is located within the premises of a company with a plurality of employees, each of them having a mobile station operating with the private base station. In this case, the private base station is also called a company or office base station.
The private base station is connected to the public telephone network. Thus, each mobile station communicates with the public network via the private base station with which it is associated. The private base station provides for local coverage only, roughly corresponding to an area the size of an apartment, a house and garden, or business premises.
Conversely, cellular systems (of the GSM 900 or DCS 1800 types for instance) are designed to provide much wider coverage, for instance the size of a country or even of a continent. Generally speaking, a cellular radio system is implemented within a geographical network of cells. A base station (hereinafter sometimes called a cellular base station) is associated with each cell. Each user has a mobile station (also called a portable terminal) capable of communicating via the base station associated with the cell within which the user is located.
Cordless and cellular systems have been developed in parallel for some considerable length of time, hence the existence of two base station designs, i.e. those usable only in a cellular system and those usable only in a cordless system.
Recently, attempts have been made to integrate both xe2x80x9ccordlessxe2x80x9d and xe2x80x9ccellularxe2x80x9d types of operation in the same mobile stationxe2x80x94called a dual mode mobile station. Indeed, because of the complexity of the cellular system, its use costs more to the user than the use of a cordless system. In other words, subscriptions and communications are charged at a higher rate in a cellular system than in a cordless system (the rates applied in the case of a cordless system being those of the public network to which the private base station is connected). Therefore, attempts are being made to make it possible for the user to use a mobile station in xe2x80x9ccordlessxe2x80x9d mode as often as possible (i.e. as soon as the user is within local coverage of the private base station), with xe2x80x9ccellularxe2x80x9d mode being used only when operation in xe2x80x9ccordlessxe2x80x9d mode is not possible.
The first known solution, which is the easiest to implement, consists in juxtaposing in the same casing the components of both a xe2x80x9ccordlessxe2x80x9d mobile station and those of a xe2x80x9ccellularxe2x80x9d mobile station.
As can be easily understood, that first solution is very expensive since the price of a dual mode mobile station implemented in that way corresponds roughly to the sum of the prices of a xe2x80x9ccordlessxe2x80x9d mobile station plus a xe2x80x9ccellularxe2x80x9d mobile station.
A second known solution, which is more advantageous, has recently been proposed. It consists in designing a private base station compatible, in terms of frequency band, with the cellular system used in xe2x80x9ccellularxe2x80x9d mode. In other words, the private base station transmits and receives over frequencies contained within the same band as the frequencies transmitted and received by base stations of the cellular system. The operation of the private base station and of the dual mode mobile station in this case is as briefly described below.
In standby mode (i.e. so long as the dual mode mobile station has not registered or after the dual mode mobile station has registered), the private base station transmits over a beacon channel to reveal its presence to the dual mode mobile station.
As for the dual mode mobile station, when in standby in xe2x80x9ccellularxe2x80x9d mode, it periodically listens to the beacon channel of its associated private base station. When the dual mode mobile station enters the local coverage of its private base station, it recognizes the beacon channel and then registers with the private base station. Following this registration, the dual mode mobile station operates in xe2x80x9ccordlessxe2x80x9d mode and can be used to directly send calls to (or receive calls from) the public network, independently of the cellular network.
It is important to note that the registration of the dual mode mobile station with the private base station constitutes a stage, executed once and for all, prior to operating in xe2x80x9ccordlessxe2x80x9d mode. In contrast, actual xe2x80x9ccordlessxe2x80x9d mode operation consists of carrying out successively, a plurality of call set up stages (incoming or outgoing), each set up stage being followed (except in case of problems) by a conversation stage. In other words, from the moment the dual mode mobile station is registered, a plurality of successive calls can be set up and a plurality of conversations can be processed, without requiring a new registration each time.
In active mode, after the dual mode mobile station has registered with the private base station, the private base station continues to transmit over the beacon channel, in order to warn the dual mode mobile station that a call is intended for it (incoming call) or that an access request previously sent by the dual mode mobile station (outgoing call) has been accepted.
It is obvious that with that known solution, the cost of the dual mode mobile station is much less than that of the first solution, since the same hardware is used in both xe2x80x9ccordlessxe2x80x9d and xe2x80x9ccellularxe2x80x9d modes.
In counterpart, with that second known solution, because of the simultaneous use of the same frequency band, there exists a risk of interference, both between various private base stations, and between private base stations and cellular base stations.
In order to minimize this risk, it has been proposed that the beacon channel be reduced to an intermittently transmitted signal, with a low operating cycle (i.e. with time slots that are relatively far apart).
Unfortunately, this proposition is not good in terms of interference. Indeed, even though the beacon channel is transmitted with a low operating cycle, it nevertheless constitutes a source of interference with all other base stations (home or cellular) using the same beacon channel. It is therefore necessary to ensure coordination in terms of frequencies and geographical coverage, but this entails very complex purchasing and installation procedures.
A particular object of the invention is to mitigate the various drawbacks of this state of the art.
More precisely, one of the objects of the present invention is to provide a registration method used for registering a dual mode mobile station with a private base station, which would enable the risks of interference to be minimized both between various private base stations and also between private base stations and cellular base stations.
The invention also aims at providing such a method, the implementation of which requires few modifications to a dual mode mobile station compared with corresponding cellular stations already in existence.
These various objects, as well as others which appear below, are achieved according to the invention by a registration method used for registering at least one dual mode mobile station with an associated private base station, said private base station being connected to the public telephone network and using, for transmission and reception, frequencies which are close to or identical to those used in a cellular radio system, said dual mode mobile station being able to operate either in xe2x80x9ccordlessxe2x80x9d mode, or in xe2x80x9ccellularxe2x80x9d mode,
said method consisting in interchanging signals in accordance with a preset protocol based firstly on a preset frequency hopping law, determining the frequency over which each signal is to be transmitted, and secondly on a preset uplink start-up frequency, over which the dual mode mobile station transmits the first signal, said frequency hopping law and said uplink start-up frequency being specific to, and known by, the dual mode mobile station and associated private base station.
said method being characterized in that it comprises an initial contact stage itself including the following steps:
the dual mode mobile station transmits a contact burst, made up of a pure sine wave at said uplink startup frequency;
the private base station, which is in receive mode over said uplink start-up frequency, receives the contact burst, and then answers by transmitting an acknowledgment burst, made up of a pure sine wave at a downlink start-up frequency associated with said uplink start-up frequency; and
the dual mode mobile station receive the acknowledgment burst.
Therefore, the general principle of the invention consists of pairing a dual mode mobile station with a private base station by making them operate a signal interchange protocol based on the same frequency hopping law, and with the same uplink start-up frequency, so that, depending on the previously transmitted or received frequency, each of the two stations (private base station or dual mode mobile station) knows whether it is to transmit or receive, and over which frequency.
Thus, because of frequency hopping, any risks of interference with other base stations (home or cellular) during the registration stage are minimized.
Besides, since the main protocol elements (i.e. frequency hopping law and uplink start-up frequency) are specific to the pair (private base station/dual mode mobile station), the duration of the registration stage can be very short.
Furthermore, it is important to note that, in the invention, it is always the dual mode mobile station that initiates the interchange of signals. Therefore, in standby mode, the private base station does not transmit any signal (it monitors the uplink start-up frequency), which prevents any interference with other base stations (home or cellular).
Advantageously, the dual mode mobile station transmits the first signal at the uplink start-up frequency after the user has indicated a desire to register the dual mode mobile station with the private base station.
Thus, it is the user who decides when to register a dual mode mobile station with the private base station. As explained above, this prevents the private base station from having to transmit a beacon-type signal in order to make its presence known to the dual mode mobile station.
Advantageously, the private base station is independent from the cellular radio system, the dual mode mobile station being accessible:
in xe2x80x9ccordlessxe2x80x9d mode, by means of a first number associated with the private base station within the public telephone network, and
in xe2x80x9ccellularxe2x80x9d mode, by means of a second number, that is distinct from said first number and that is associated, in the cellular radio system, with a subscriber identification module inserted in the dual mode mobile station.
In other words, in this instance, no attempt is made to keep the same number for both the xe2x80x9ccordlessxe2x80x9d and the xe2x80x9ccellularxe2x80x9d modes. It is important to note that, in the invention, operation in xe2x80x9ccordlessxe2x80x9d mode occurs completely independently of the cellular system (whether it be for making contact (with synchronization) or for setting up a call).
Generally speaking (and particularly in the case of the GSM), the subscriber""s cellular number is stored in a card (for example a smart card) containing data which is specific to the subscriber. This card, called subscriber""s identification module (or SIM card) in GSM, is designed to be inserted in any mobile station, and therefore, in particular, in the dual mode mobile station. In contrast, the subscriber""s public number is always associated with a specific telephone connection to which the private base station is connected.
Preferably, the frequencies transmitted by the dual mode mobile station and by the private base station in application of said preset frequency hopping law are frequencies that are also used in xe2x80x9ccellularxe2x80x9d mode by the dual mode mobile station and by base stations within the cellular system,
each xe2x80x9cuplinkxe2x80x9d frequency transmitted by the dual mode mobile station being associated with a xe2x80x9cdownlinkxe2x80x9d frequency transmitted by the private base station, in order to make up one of the bi-directional channels used by the dual mode mobile station and the private base station in application of said preset frequency hopping law.
In other words, the frequency hopping law serves to switch from one bi-directional channel to another, each channel including an uplink (from dual mode mobile station t o private base station) and a downlink (from private base station to dual mode mobile station). The conventional channel structure of cellular systems (especially of the GSM type) is thus used, thereby avoiding hardware modifications to the dual mode mobile station compared with conventional (cellular only) mobile stations.
Preferably, said preset frequency hopping law belongs to the group containing:
hopping laws conventionally used in cellular radio;
pseudorandom hopping laws; and
hopping laws minimizing the probability of interference between cellular system and cordless system.
In an advantageous method of implementation of the invention, at least two dual mode mobile stations are associated with said private base station, the private base station and all dual mode mobile stations use the same preset uplink start-up frequency, and each distinct pair (private base station/dual mode mobile station) uses a distinct, preset frequency hopping law.
In this way, all aforementioned advantages also apply to the case of a private base station associated with a plurality of dual mode mobile stations.
This method of implementation using a plurality of dual mode mobile stations (operating with the same private base station) corresponds in particular to a business installation, where several employees possess dual mode mobile stations allowing them to be connected to the company""s private base station.
In preferred manner, when the aforesaid radio system is of the time-division multiple access type, with the interchanged signals being structured in successive frames, and with each frame containing a preset number of time slots, said frame structure is preserved, each dual mode mobile station being allocated a specific time slot rank in each frame.
In a particular implementation of the invention, the aforesaid method includes the following successive stages:
an initial contact stage, during which the private base station and the dual mode mobile station check that they can contact each other;
a time synchronization stage, during which the dual mode mobile station is synchronized in time with the private base station;
a standby stage, during which:
the private base station and the dual mode mobile station ensure that their mutual time synchronization is maintained;
during an outgoing call, the dual mode mobile station transmits to the private base station, an access signal for the purpose of setting up a call;
during an incoming call, the private base station transmits to the dual mode mobile station, a paging message for the purpose of setting up a call.
It should be noted that, with this particular implementation method, the dual mode mobile station and the private base station synchronize themselves mutually, and do so completely independently of the cellular system.